An urgent need has existed, for at least the last decade, for a reliable, high-quality, autonomous device to record and store dynamic blast effects data in severe inertial fields. Current and future weapons effects research requires that measurements be made very near the explosive source, where shock levels commonly approach and sometimes exceed 100,000 g's. Conventional, wired sensor signal cables exposed to these environments almost invariably fail, at first shock arrival or shortly thereafter, from effects of high thermal flux and cable shearing due to rapid differential displacement.
In the prior art, to measure shock levels and air pressures resulting from shock waves generated as a result of an explosive blast, cables connecting the measurement transducer(s) and its signal conditioning modules, which ordinarily are rack mounted in an instrumentation vehicle (van), with one module for each transducer, are used. It is in the instrumentation vehicle that a shunt calibration resistor is mounted in series with a set of relay contacts. When the relay contacts are energized, a calibration resistor (used in a calibration step of a signal conditioning unit) shunts one arm of the bridge of the transducer. The calibration step, and the subsequent data, are recorded by either an analog tape machine or a transient data recorder. Both the signal conditioning and record units require large amounts of space and considerable power. Furthermore, they must be protected from the environment to the extent that the vehicle must be air conditioned to maintain a constant temperature. In addition, these units are not designed to withstand shock levels exceeding 50 g's, and therefore must be located hundreds to thousands of feet from the blast.
Therefore, to record the shock or air pressure levels, the prior art method requires that cables be used between the transducer and the rest of the system. This places a limit on how close to the detonation measurements can be made, as the survivability of the cables decrease as the measurement point nears ground-zero. On large explosive tests, this distance becomes significant, so much so that complete and definitive measurements within the ground-zero region of these tests have yet to be made.
Thus, the measurement of air pressure levels and shock levels, i.e. high pressures, and accelerations (ground motion) in close proximity to ground-zero (the location of the explosive), has been limited because of the destruction of the data cables early in the explosion phase of the tests.
This limitation aside, the use of long cables becomes disadvantageous because of the inherent significant cable resistance and capacitance which tend to place severe restrictions on transient data fidelity and calibration data validity.
So, too, the frequency response of the cables should be equal to or better than the frequency response of the transducer that is providing the data. That is, a typical waveform is made up of both low and high frequency components, and the cables should be able to transmit the waveform without distorting it. However, the distributed capacitance associated with long cables acts as a low-pass filter that attenuates the amplitude of the high frequency components and shifts their phase (delays them in time). The low frequency components of the waveform, on the other hand, are unaffected. The frequency range of the cables can be extended by "swamping" the output of the cables with a low resistance value. However, a very high penalty is paid, as the amplitude of the entire signal is reduced, thereby decreasing the signal to noise ratio.
Further, long cables attenuate the signals generated by a transducer. As mentioned above, this attenuation is exacerbated by the low resistance termination, which is needed to improve the frequency response of the cables. Added to this is the requirement that the frequency response of the amplifier must also be equal to or better than the frequency response of the transducer. Moreover, the added cable attenuation requires increasing the gain of the amplifier. However, increasing the gain of the amplifier decreases its frequency response. Amplifiers with no bandwidth to spare will then distort the signals. The use of wideband amplifiers, operated at high gain, are then required to faithfully reproduce signals with high frequency components. Yet such wideband amplifiers used in prior art methods are closely stacked. And when operated at high gains, these amplifiers tend to crosstalk. In addition, if one of the amplifiers has a tendency to oscillate, the whole rack of such stacked amplifiers will likely oscillate in sympathy.
Still another problem with long cables is that they introduce errors in the scaling of the calibration pulse generated by the application of shunt calibration resistors across the arm of the transducer bridge. The principle applied here is that a resistance of proper value, so placed, unbalances the bridge by an amount equivalent to that caused by a known value of the measurand. This in turn provides the key value used to scale the entire waveform. On long lines the accumulated resistance and circuit configuration are such that large errors can be generated and must be factored out. The usual method of doing this is to measure the line resistance and place these values in an appropriate formula to calculate the correction factor. These additional data measurement and calculations (one for each transducer) is both time consuming and error vulnerable.
Yet still another problem with long cables is that they act as antennas and are susceptible to noise and crosstalk. This is especially true when they are run in parallel, which usually cannot be avoided for explosive tests. The majority of the transducers are located near ground zero, and all of the cables must terminate in the instrumentation vehicle that carries the equipment for the measurements. In an active electromagnetic environment where many researchers have instrument trailers feeding respective transducers in a common region, data transmission interactions can and do occur when the equipment is operating at full power and full frequency. Even when this interference is not noticed on the final dry run, any last minute changes introduced before zero-time can contaminate the data of the more susceptible systems. Again, the worst case situation is when all of the cables are run in parallel, which often cannot be avoided. For example, many of the tests are done in tunnels or underground; and there is usually a common point where all of cables enter the test region.
Another further disadvantage of employing long cables is their susceptibility to developing destructive surge currents that are induced by potential lightning discharges at the test site. It has been known that several major explosive tests have been postponed and reinstrumentated at great expense because of this phenomenon.
Lastly, procurement of miles of cables is very expensive and installation costs are also high.
Cables aside, another hurdle that needs to be surmounted is the required portable nature of the data acquisition system, particulary in view of the fact that such data acquisition system oftentimes has to measure the impact levels and acceleration of a moving vehicle, such as for example a projectile in flight.
In view of the many aforenoted disadvantages, there is a need for a data acquisition system that can operate independently of previously required long cables and yet remain substantially unaffected by shock levels and pressures from an explosive source. Also, a need exists for a small, low mass measurement/acquisition device that can be carried on board of a moving projectile and be able to operate in an extremely high inertial environment. Needless to say, such data acquisition system must also be able to continue recording the shock levels and/or the air pressures of a ground level explosion, and the impact levels and acceleration were the system placed aboard a projectile, notwithstanding the many possible thousands of g's that may accompany such explosion and/or the movement and impact of the projectile.